In a conventional power conversion circuit where a free-wheeling diode is connected in inverse parallel to a switching element in a main circuit, what have been desired are a semiconductor switch that decreases a loss caused by the flow of a reverse recovery current in the free-wheeling diode and a power conversion apparatus that includes the semiconductor switch.
In a semiconductor switch where a high-speed free-wheeling diode is connected in inverse-parallel to a switching element, when a main current is flowing in the reverse direction, if the semiconductor switch is turned off (or when a main element and an auxiliary element are turned off), the voltage remaining between both ends of the main element is determined by the product of the inductance value of a circuit loop that connects the main element, auxiliary element, and high-speed free-wheeling diode and the magnitude of the main current. To make faster the switching speed of the semiconductor switch, it is necessary to make the inductance of the circuit loop as low as possible. Therefore, the voltage remaining in the main element sometimes dropped as low as below several voltages.
Here, it is necessary to consider the output capacitance of the main element when the element is off. For instance, such a semiconductor switching element has the property of including a very large capacitance if the residual voltage applied to the element in the off state is low. The reason is that the depletion layer at the junction part is thin when the output terminal voltage is low and therefore the element has a very large capacitance.
When the capacitance is very large, if one semiconductor switch that operates complementarily is turned on in a bridge circuit composed of two semiconductor switches connected in series in a direct-current main circuit, a charging current will flow toward the large capacitance. Since the circuit impedance is very low, the charging current might become a large current as in a short circuit, causing a large loss.